Criminal law involves prosecution by the government of a person for an act that has been classified as a crime. Civil cases, on the other hand, involve individuals and organizations seeking to resolve legal disputes. In a criminal case, the state, through a prosecutor, initiates the suit, while in a civil case the victim brings the suit. Persons convicted of a crime may be incarcerated, fined, or both. However, persons found liable in a civil case may only have to give up property or pay money, but are not incarcerated.
A "crime" is any act or omission (of an act) in violation of a public law forbidding or commanding it. Though there are some common law crimes, most crimes in the United States are established by local, state, and federal governments. Criminal laws vary significantly from state to state. There is, however, a Model Penal Code (MPC) which serves as a good starting place to gain an understanding of the basic structure of criminal liability.
Crimes include both felonies (more serious offenses -- like murder or rape) and misdemeanors (less serious offenses -- like petty theft or simple battery). Felonies are usually crimes punishable by imprisonment of a year or more, while misdemeanors are crimes punishable by less than a year. However, no act is a crime if it has not been previously established as such either by statute or common law. Recently, the list of Federal crimes dealing with activities extending beyond state boundaries or having special impact on federal operations, has grown. See Title 18.
All statutes describing criminal behavior can be broken down into their various elements. Most crimes (with the exception of strict-liability crimes) consist of two elements: an act, or "actus reus," and a mental state, or "mens rea". Prosecutors have to prove each and every element of the crime to yield a conviction. Furthermore, the prosecutor must persuade the jury or judge "beyond a reasonable doubt" of every fact necessary to constitute the crime charged. In civil cases, the plaintiff needs to show a defendant is liable only by a "preponderance of the evidence," or more than 50%.
Definition from Nolo’s Plain-English Law Dictionary
Laws written by Congress and state legislators that make certain behavior illegal and punishable by fines and/or imprisonment. Criminal law also includes decisions by appellate courts that define crimes and regulate criminal procedure in the absence of clear legislated rules. By contrast, civil laws are not punishable by imprisonment. In order to be found guilty of a criminal law, the prosecution must show that the defendant intended to act as he did; in civil law, you may sometimes be responsible for your actions even though you did not intend the consequences. For example, civil law makes you financially responsible for a car accident you caused but didn't intend.
Definition provided by Nolo’s Plain-English Law Dictionary.